1. Field of the Invention
This invention relates to optical detection systems for displacement tracking and more particularly to a signal processing system useful in postal scale applications.
2. Brief Description of the Prior Art
Optical detectors have been used in weighing scales for the purpose of measuring scale tare structure displacement to thus provide an indication of the mass of a load supported by the tare structure. Generally, these detectors sensed the movement of one of a pair of parallel optical gratings to provide an optically amplified function of the tare structure deflection. An example of a typical scale optical detector is disclosed in the following pending application: Weighing with Moire optoelectronic Transducer, Ser. No. 653,850, filed Jan. 30, 1976 now U.S. Pat. No. 4,044,847 and assigned to the assignee of the present invention.
Signal processing systems for tracking the optically detected deflection have utilized phase quadrature displaced pulse waveform signals generated from photodetector signals. In copending application Ser. No. 715,883 filed Aug. 19, 1976 entitled Counting System, also assigned to the assignee of the present invention, an exemplary signal processing system for scale applications is shown.
Among the problems encountered in the design of production scale systems employing optical detectors and counting circuitry was that, although high resolution optical gratings were readily available, true signal resolution was dependent on allowable air gap dimensions between the movable and fixed gratings. Air gap clearance and tolerance were greatly reduced with increased optical grating resolution. This problem was not satisfactorily alleviated in the presence of collimated light source. As a result of critical air gap dimensions, optical detectors became difficult to set up and maintain. Thus, with potential accuracies of 20 microinches available over an inch of displacement, practical optical resolution due to allowable air gap considerations limited such resolution to approximately 800 to 1000 microinches.
Attempts at increasing resolution through the use of quadrature counts derived from detected waveforms did not produce adequate results and introduced sources of additional error into the system.
The use of optical detectors having relatively low resolution, yet which maintained high accuracy, had potential acceptability in decision applications such as postal scales wherein digital countup and countdown waveforms having, for example, one ounce resolution per waveform cycle could be utilized. A major drawback remained, however, in that scales were subject to both zero displacement drift and decision edge oscillation, and for such reasons a higher degree of resolution than provided by the digital waveforms derived from the optical system was required.